Fallback DPI detection on X11

When the Xft.dpi setting is not found, and if only one screen is
available, fall back to detecting the screen DPI from the reported
resolution and monitor size.

Change-Id: I1481d94b5dc4072c1f8da4659a221dfc7971080b
Reviewed-on: https://gerrit.libreoffice.org/11377
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>